Usually if bass starts panning or feeling uncentred is due to seal issues,(even though you think you have seal) of course taking into account the bass in that track is centered. It's mostly noticable with sub-bass hits you'll feel the bass pull to one side. I have a track I use with a low sub-bass drop that I know should be centre channel bang smack in the middle. if your bass is leaning to one direction check your tips because our ears aren't exactly the same on both sides.
yeah that's true... let me check this and give ya an update :)